1. Field of the Invention
The present invention relates to a two-port isolator and, more particularly, to a two-port isolator for use in, for example, a microwave band, and also relates to a communication device provided with the two-port isolator.
2. Description of the Related Art
Generally, isolators pass signals only in the transmission direction and suppress signals in the opposite direction. Such isolators are used in transmitting circuits in mobile communication devices such as car phones and mobile phones.
Three-port isolators (isolators having first through third center electrodes) are taught in Japanese Unexamined Patent Application Publication Nos. 2001-320205, 2001-320206, 11-308013, and 2000-114818. Also, two-port isolators (isolators having first and second center electrodes) are taught in Japanese Unexamined Patent Application Publication Nos. 2001-237613 and 2001-185912.
Three-port isolators and two-port isolators propagate signals from an input port P1 to an output port P2 by magnetic coupling, and thus undesirably have a large coupling loss between the input port P1 and the output port P2.
In order to solve this problem, a low-loss two-port isolator is taught in Japanese Unexamined Patent Application Publication No. 9-232818. FIG. 21 is an equivalent circuit diagram of this conventional two-port isolator. One end 321a of the first center electrode 321 is electrically connected to the input outer electrode 314 through the input port P1. The other end 321b of the first center electrode 321 is electrically connected to the output outer electrode 315 through the output port P2. Directly connecting the input port P1 to the output port P2 through the first center electrode 321 decreases the coupling loss between the input port P1 and the output port P2.
One end 322a of the second center electrode 322 is electrically connected to the output outer electrode 315 through the output port P2. The other end 322b of the second center electrode 322 is electrically connected to the ground electrode 316 through the third port P3. A parallel RC circuit including a matching capacitor 325 and a resistor 327 is electrically connected between the input port P1 and the output port P2. A matching capacitor 326 is electrically connected between the output port P2 and another ground electrode 316. The ground electrodes 316 are electrically grounded.
The input port P1 is directly connected to the output port P2 through the first center electrode 321 in the known two-port isolator 301, thus disadvantageously propagating the second harmonic wave (2f) or the third harmonic wave (3f) of the frequency f used in the mobile communication device.